Or if there were a duplicate of Earth around a particular star, could it be detected with our equipment?
Actually imaging Earth-like planets may someday be done with massive interferometers in orbit. I'd consider O2 in the atmosphere a sign of life.
We can't forget that Earth is 4.5 billion years old, over half that time there were no eukaryotes, big animals date from the half a billion ya, and we've only been broadcasting radio for 100-odd years.
Here's a link to one of RA's more technical explanations of how the distance over which one can detact a signal is calculated. You'd have to fill in values for the various factors and do the calculation to see what the answer would be. Heavy GEEK ALERT:
http://www.freerepublic.com/focus/f-news/1289285/posts?page=28#28